The use of fuel cells to generate electrical power for electricity or to drive a transportation vehicle relies upon the generation of hydrogen. Because hydrogen is difficult to store and distribute, and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline. To be effective, hydrogen generation for fuel cells must be smaller, simpler, and less costly than hydrogen plants for the generation of industrial gasses. Furthermore, hydrogen generators for use with fuel cells will have to be integrated with the operation of the fuel cell and be sufficiently flexible enough to efficiently provide a varying amount of hydrogen as demand for electric power from the fuel cell varies. Large-scale hydrogen plants and related facilities have very sophisticated process control systems related to controlling the process to achieve efficient operation and to provide safe start-up, shut-down, and normal operating responses to upsets in feed or product variations.
Hydrogen is widely produced for chemical and industrial purposes by converting materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas. Such production usually takes place in large facilities which are rarely turned down in production for even a few days per year. In addition, the operation of the industrial hydrogen production facilities are often integrated with associated facilities to improve the use of energy for the overall complex. Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen. It is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, autothermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream. A paper titled "Will Developing Countries Spur Fuel Cell Surge?" by Rajinder Singh, which appeared in the March 1999 issue of Chemical Engineering Progress, page 59-66, presents a discussion of the developments of the fuel cell and methods for producing hydrogen for use with fuel cells. The article particularly points out that the partial oxidation process is a fast process permitting small reactors, fast startup, and rapid response to changes in the load, while steam reforming is a slow process requiring a large reactor and long response times, but operates at a high thermal efficiency. The article highlights one hybrid process which combines partial oxidation and steam reforming in a single reaction zone as disclosed in U.S. Pat. No. 4,522,894.
Modifications of the simple steam reforming processes have been proposed to improve the operation of the steam reforming process. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from the products of a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No.4,479,925 in which heat from the products of a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula: EQU CH.sub.4 +2H.sub.2 O.fwdarw.4H.sub.2 +CO.sub.2
where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following simplified formulae (1) and (2): EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 (1) EQU CO+H.sub.2 O.fwdarw.H.sub.2 +CO.sub.2 (2)
In the water gas shift converter which typically follows a reforming step, formula (2) is representative of the major reaction.
U.S. Pat. No. 4,925,456 discloses a process and an apparatus for the production of synthesis gas which employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-reactor and the primary reforming catalyst is positioned either in the central core or in the annulus thereof. The invention is further characterized in that the secondary reformer effluent is passed through which ever of the central core or the annulus is not containing the primary reforming catalyst countercurrently to the hydrocarbon-containing gas stream.
U.S. Pat. No. 5,181,937 discloses a system for steam reforming of hydrocarbons into a hydrogen rich gas which comprises a convective reformer device. The convective reformer device comprises an outer shell enclosure for conveying a heating fluid uniformly to and from a core assembly within the outer shell. The core assembly consists of a multiplicity of tubular conduits containing a solid catalyst for contacting a feed mixture open to the path of the feed mixture flow such that the path of the feed mixture flow is separated from the heating fluid flow in the outer shell. In the process, an autothermal reformer fully reforms the partially reformed (primary reformer) effluent from the core assembly and supplies heat to the core assembly by passing the fully reformed effluent through the outer shell of the convective reforming device.
U.S. Pat. No. 5,595,833 discloses a process and apparatus for operating a solid oxide fuel cell stack and includes an adiabatic pre-reformer to convert about 5 to 20% of the hydrocarbon fuel into methane, hydrogen, and oxides of carbon At startup the pre-reformer is used to perform partial oxidation with methanol to heat the solid oxide fuel stack to a temperature of about 1000.degree. C. When the temperature of the region of the pre-reformer reaches about 500.degree. C. the methanol flow is terminated.
WO 97/45887 discloses a hydrodesulfurizer assembly which is thermally coupled with process gas heat exchangers and a shift converter. The hydrodesulfurizer assembly is employed to cool the reformer effluent prior to passing the cooled reformer effluent to the shift converter zone.
WO 98/13294 discloses a process for removing carbon monoxide from a gas stream by subjecting the gas stream to a first stage high temperature selective catalytic methanation to lower the carbon monoxide concentration, followed by a second stage low temperature selective catalytic methanation to further lower the residual carbon monoxide concentration in the gas stream to a carbon monoxide concentration below 40 ppm.
U.S. Pat. No. 4,943,493 discloses a fuel cell power plant which integrates the operation of a reformer to convert a hydrocarbon fuel into a hydrogen-rich fuel which is passed to the anode side of a fuel cell. A portion of the anode exhaust stream is withdrawn from the fuel cell and passed to a burner zone wherein the anode exhaust gas stream is mixed with an oxidant stream and combusted to provide heat to the reformer. U.S. Pat. No. 4,943,493 discloses the problem of monitoring and controlling the flame temperature in the burner zone and claims an indirect approach to maintaining the flame temperature with a range which results in complete combustion of the fuel and avoids a very high flame temperature which may exceed the temperature resistance of the burner liner materials.
The reference discloses the control of the composition of the burner gas by bypassing a portion of the anode waste gas to maintain an adiabatic flame temperature between about 1150.degree. C. (2100.degree. F.) and about 1480.degree. C. (2700.degree. F.) whereby the heat transfer to the reforming zone occurs in the radiant region to provide a high efficiency steam reforming operation.
U.S. Pat. No. 4,861,348 discloses a fuel reforming apparatus wherein the heat for the reforming zone is provided by a combustor. Flames formed within the combustion zone generate a high temperature combustion gas. The apparatus includes a heat-insulating layer for preventing radiation heat losses from the combustion gas, and a combustion gas passage disposed around the reforming zone to permit combustion gas to flow therethrough. A hydrocarbon/steam mixture is preheated by flowing on the outside of the combustion gas passage in a supply passage before the mixture is passed to the reforming zone. Heat insulation is provided as an outer layer disposed around the outer peripheral surface of the supply passage to prevent the loss of radiation from the inner wall. In one embodiment, reforming catalyst is disposed on the outside of the combustion gas passage in the supply passage to extend the reforming zone.
U.S. Pat. No. 4,863,712 discloses a steam reforming process wherein a hydrocarbon feedstock, such as methane, natural gas, LPG, or naphtha is reacted with steam and/or carbon dioxide in the presence of a supported catalyst such as nickel or cobalt. The heat required for the endothermic reaction is supplied from the sensible heat of the reactants or from an external heat source. The reformer outlet is maintained in the range of 700-900.degree. C. or higher.
U.S. Pat. No. 4,869,894 discloses a process for the production and recovery of high purity hydrogen. The process comprises reacting a methane-rich gas mixture in a primary reforming zone at a low steam-to-methane molar ratio of up to about 2.5 to produce a primary reformate, followed by reacting the primary reformate in a secondary reforming zone with oxygen to produce a secondary reformate, comprising hydrogen and oxides of carbon. The secondary reformate is subjected to a high temperature water gas shift reaction to reduce the amount of carbon monoxide in the hydrogen-rich product. The hydrogen-rich product is cooled and processed in a vacuum swing adsorption zone to remove carbon dioxide and to produce a high purity hydrogen stream.
WO 98/08771 discloses an apparatus and method for converting feed streams such as a hydrocarbon fuel or an alcohol into hydrogen and carbon dioxide. The process comprises passing the feed stream first to a partial oxidation reaction zone to produce a partial oxidation effluent. The partial oxidation effluent is passed to a separate steam reforming reaction zone. The partial oxidation reaction zone and the steam reforming reaction zone are disposed in a first vessel. A helical tube is extended about the first vessel and a second vessel is annularly disposed about the first vessel such that the helical tube is disposed between the first and second vessels. The third vessel annularly disposed around the second vessel. Oxygen is preheated in the helical tube by heat from the partial oxidation reaction prior to being passed to the partial oxidation zone. The reformate from the steam reforming reaction zone is passed between the first and second vessel and is subjected to a high temperature shift reaction to reduce the carbon monoxide content of the reformate stream. The thus treated reformate stream is desulfurized, cooled, and subjected to a low temperature shift reaction.
U.S. Pat. No. 5,741,474 discloses a process for producing high purity hydrogen by reforming a hydrocarbon and/or oxygen atom containing hydrocarbon to form a reformed gas containing hydrogen, and passing the reformed gas through a hydrogen-separating membrane to selectively recover hydrogen. The process comprises the steps of heating a reforming chamber, feeding the hydrocarbon along with air and/or steam to the chamber and therein causing both steam reforming and partial oxidation to take place to produce a reformed gas. The reformed gas is passed through a separating membrane to recover a high purity hydrogen stream and the non-permeate stream is combusted to provide heat to the reforming chamber.
U.S. Pat. No. 5,858,314 discloses a natural gas reformer comprising a stack of catalyst plates supporting reforming catalyst and a plurality of thermally conducting plates alternately stacked to form a reforming structure, wherein the conductive plates transfer heat energy in-plane, across the surface of the conductive plate to support the reforming process.
Conventional steam reforming plants are able to achieve high efficiency through process integration; that is, by recovering heat from process streams which require cooling. In the conventional large-scale plant this occurs in large heat exchangers with high thermal efficiency and complex control schemes. In the present invention for the production of hydrogen for fuel cells it is desired to reach a high degree of process integration, with minimal equipment in order to reduce the size of the plants and the complexity of the control scheme. U.S. Pat. No. 5,861,441 discloses a process that is representative of such an integrated processing scheme for large plants with integrated compression and heat exchange. It is the objective of this invention to provide a compact apparatus for generating hydrogen from available fuels such as natural gas, hydrocarbons, and alcohols for use in a fuel cell to generate electric power. One of the problems faced by developers of hydrogen generators working with fuel cells for domestic and transportation use is the high cost of exotic material of construction which are required to withstand the high reaction temperatures of the partial oxidation and reforming processes. It is an objective of the present invention to provide a hydrogen generator for converting natural gas to hydrogen which can be operated without exceeding a process temperature of 700.degree. C. in the heat exchange equipment and thus can be constructed of conventional materials.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Polymers with high protonic conductivities are useful as proton exchange membranes (PEM's) in fuel cells. Among the earliest PEM's were sulfonated, crosslinked polystyrenes. More recently sulfonated fluorocarbon polymers have been considered. Such PEM's are described in an article entitled, "New Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymers", by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
It is an objective of the present invention to solve some of the problems associated with small-scale systems for producing hydrogen for a fuel cells, to provide simplified methods for producing hydrogen for a fuel cell, to provide simple and efficient methods for controlling the hydrogen generation system associated with a fuel cell, and to provide an apparatus for the generation of hydrogen that permits the reduction in scale of hydrogen generation facilities without a corresponding loss of efficiency. It is an objective of the present invention to provide a process for using the anode waste gas as the primary fuel for the generation of hydrogen for a fuel cell wherein the fluctuations in the anode waste gas flow rate and heating value are managed in the process to maintain a high overall energy efficiency.
It is an objective of this invention to provide an integrated fuel cell and hydrogen production system which is energy and hydrogen efficient. More particularly, it is an objective of the present invention to provide a process which starts up rapidly while operating at an efficiency level approaching that of a steam reformer operation.
It is an objective of the present invention to provide a process for the generation of hydrogen for use in a fuel cell which offers a high degree of feed flexibility and which eliminates the use of a separate external fuel.
It is an objective of the present invention to provide a process and control system which avoids thermal cycling of the heat transfer equipment in a hydrogen generator for a fuel cell for the generation of electricity. Thermal cycling in heat exchange and reactor equipment can have a deleterious effect on such equipment and result in pre-mature equipment failure. In the operation of fuel cells the demand for electricity is generally not constant resulting in the turn-down of the hydrogen generation equipment. Typically the turn-down ratios are very large in proportion to the daily fluctuation of demand for electric power. In addition, such systems often experience variation in the supply and quality of feeds and fuels consumed in the process which can impart a thermal cycling of the heat exchange equipment. Such thermal cycling can damage welds which could compromise the safety of the operation of the hydrogen generation equipment.